Our ISP at work is TelstraClear (our external IP address is 203.97.27.4, which according to APNIC is owned by them).

For the past two months, we have sporadically incredibly poor performance as soon as some international sites hit a content distribution network.

Examples:

apple.com nytimes.com engadget.com msdn.microsoft.com

As soon as it tries to load, CSS, JS or images from these CDN servers, it stalls for minutes, eventually timing out.

How would you suggest I go about trying to prove its TelstraClear at fault?

I'm fairly certain it has to be, other international sites which do not use the same CDNs load fine, Telecom 3G works fine, Telecom broadband works fine, at these same times.

Work IT guy says it's them, something to do with their locally hosted cache servers being fubar, but how would you recommend I go about proving this in a way that will get them to pull their fingers out?

Is it:

Transparent proxy server that they can't manage to scale (this was an issue in 2003...)

Transparent DNS that they're still screwing up (this was in issue in 2003, if it's still going....)

Local cache servers for Akamai or other networks that they can't keep up for more than minutes at a time